α-Receptor and cholinergic receptor-linked changes in cytosolic Ca 2+ and membrane potential in primary rat astrocytes

Abstract

Both phenylephrine and carbachol caused a sustained increase in Ca 2+ influx and intracellular free Ca 2+ of primary astrocytes as measured with 45Ca 2+ and fura-2. The responses to phenylephrine and carbachol were additive, suggesting that they use different releasable pools of Ca 2+. If extracellular Ca 2+ was removed by EGTA only a transient rise in cytosolic Ca 2+ was seen upon application of the agonists. Both compounds caused depolarization of the astrocyte membrane as determined with the optical probe 3,3-diethylthiadicarboxyamineiodide. Activation of protein kinase C with 12-tetradecanoylphorbol myristate acetate (TPA) or the diacylglycerol analogue dioctanoylglycerol (DiC 8) also depolarized the cells. A prior activation of protein kinase C with TPA or DiC 8 abolished the depolarizing effect of phenylephrine suggesting that they act through the same mediators. If the cells were made ideally permeable to K + with the ionophre valinomycin, or the K + channels had been blocked with Ba 2+, neither TPA nor phenylephrine had any significant effect on the membrane potential. Neither TPA nor phenylephrine had any effect on the 86Rb + equilibrium potential across the cell membrane. The results suggest that the depolarizing effect of these substances could be through a blocking of K + channels.